1. Field Of The Invention
The present invention provides a method of opening and extending fractures in a subterranean formation surrounding a wellbore penetrating the formation. The invention is particularly useful in formations which are naturally fractured such as coal seams, shales and chalk formations.
2. Brief Description Of The Prior Art
In many types of wells penetrating subterranean formations a casing is placed in the borehole and the casing then is perforated to establish communication between the wellbore and the subterranean formation. The casing typically is cemented in place within the borehole. The formation of perforations in the casing preferably establishes communication through the casing and surrounding cement into the adjacent subterranean formation. It is often desirable to fracture the subterranean formation in contact with the perforations to thereby facilitate the flow of any hydrocarbons or other fluids present in the formation to the wellbore.
Various methods and apparatus have been used to effect perforation of a well casing and fracturing of a subterranean formation. Perforations have been produced mechanically such as by hydrojetting and through the use of explosive charges such as in jet perforating. Fracturing has been accomplished by introducing an aqueous or hydrocarbon liquid into the formation through the perforations at a rate and pressure sufficient to fracture the subterranean formation. In some instances, the fracturing fluid may include a propping agent to prop the created fracture open upon completion of the fracturing treatment. The propped fracture provides an open channel through which fluids may pass from the formation to the wellbore.
Fracturing also has been accomplished by the detonation of explosives within a portion of a wellbore or the ignition of a quantity of a combustible gas mixture confined within a wellbore which produces a high pressure wave that fractures the formation surrounding the wellbore. Combustible or explosive liquids also have been utilized to fracture a subterranean formation. In this instance, the liquid reactants are injected into a wellbore and into the adjacent porous portions of the formation after which the liquid reactants are detonated to produce fractures in the formation.